The state of the present art prior to the present invention is distinguished by valves wherein the closure and opening of the oil return line from the lubricating circuit of the machine to the feed tank is achieved by axial displacement of a rubber stopper which permits the oil to travel only upstream-to-downstream and not in the reverse direction. Each of these prior valves is generally applicable to a single type of system with preset pressure and does not permit smooth starting of the pump.
This prior state-of-the-art has some deficiencies and disadvantages deriving from the fact that these rubber elements, aside from being occasionally subject to attack and deterioration by the oil, are also subject to wear and permanent deformation and to breakage in the location -- itself subject to high stresses sometimes of the alternating type -- intended for closure of the oil return line to the tank. Moreover, the traditional valves are not provided with regulating means for oil return to the tank, thus causing an overload on the pump when it is started and sometimes preventing zeroing of the pressure in the lubricating circuit when feed ceases to achieve the necessary return to the initial position of the dispensing valves operating at various points in the circuit. Finally, the conventional oil circuit valves are provided with auxiliary valve elements to permit opening of the oil return line to the tank by the traditional plugs being pushed by a spring toward such line.
SUCH DEFICIENCIES AND DISADVANTAGES DICTATE THE NEED FOR A SOLUTION TO THE NEW TECHNICAL PROBLEM OF FINDING A VALVE WHEREIN THE CLOSING ELEMENT OF THE OIL RETURN LINE ENABLES HIGH LOADS TO BE TOLERATED WITH NO DAMAGE, ENSURING RELATIVELY PERFECT RETENTION OF THE OIL EVEN AT LOW PRESSURE.